This indicates an attack attempt against a Buffer Overflow vulnerability in Losant Arduino MQTT Client.
The vulnerability is due to insufficient sanitizing of inputs when the vulnerable application handles a maliciously crafted request. A remote authenticated attacker could potentially exploit this vulnerability via a crafted request.
Losant Arduino MQTT Client prior to V2.7
Denial of Service: Remote attackers can crash vulnerable systems.
Apply the most recent upgrade or patch from the vendor.